1. Field of the Invention
This invention is related generally to condenser-type bushings used on high-voltage electrical equipment and more specifically to a means for securing the condenser portion of the bushing.
2. Description of the Prior Art
High-voltage electrical equipment requires a through electrical connector to connect the internals of the equipment to the outside world. For decades electrical bushings have provided this connection. Bushings are well known and their characteristics well understood in the prior art.
An electrical apparatus typically has an opening in its shell, casing, or tank. This opening is provided with a means for cooperating with a mounting flange of a bushing. A typical bushing has, in addition to the mounting flange, an upper weather casing and a lower casing. The upper weather casing is connected to the mounting flange such that the upper weather casing extends outwardly from the electrical apparatus. An upper electrical connector is located at the far end of the upper weather casing for connection to high-power transmission lines or the like. The lower casing of the bushing extends inwardly into the electrical apparatus and has a lower electrical connector located at one end for connection to electrical windings or the like inside the electrical apparatus. A central electrode is supported by the upper weather casing and connects the upper electrical connector and the lower electrical connector thus providing an electrical path between the outside world and the internals of the electrical apparatus. The central electrode supports the lower casing.
The central electrode is at a high-voltage potential and carries large electrical currents producing a strong electric field. In contrast, the shell of the electrical apparatus is at ground potential. Since the central electrode is in close physical proximity to the grounded shell there exists a large potential gradient therebetween. The central electrode of some types of bushings is wound with oil-impregnated paper having metallic foils at regular intervals thus producing a plurality of capacitors serially connected. The capacitors uniformly distribute the electrical stresses throughout the insulating structure of the bushing, preventing stress concentrations which can cause a discharge or flashover from the central electrode to the grounded shell. Such bushings are called capacitor or condenser bushings.
In the typical condenser bushing problems are encountered should the upper weather casing fail mechanically due to high winds, lightning strokes, a hunter's bullet, etc. Upon mechanical failure of the upper weather casing the lower casing, together with the central electrode and the condenser, may fall into the electrical apparatus. The central electrode and the condenser, being very heavy, will cause impact damage to the electrical apparatus mounted below it and will disrupt the carefully balanced electric field. This damage and field distortion may ultimately cause the electrical apparatus to fail.
Several schemes for preventing the central electrode and the condenser from falling into the electrical apparatus are known. One method is to connect the lower casing directly, and independently, to the mounting flange. Utilizing this method it becomes necessary for both the upper weather casing and the lower casing to fail before the central electrode and the condenser will fall into the electrical apparatus.
A second method is to connect the condenser directly to the mounting flange or a supporting bracket. The central electrode and condenser are thus supported independently of the upper and/or lower casings.
A third method is to wind the condenser such that its outside diameter at a point above the mounting flange is larger than the inside diameter of the mounting flange thereby preventing it from falling into the electrical apparatus.
All three of the above-mentioned methods require some modification of the typical bushing and condenser arrangement discussed above wherein the upper weather casing supports the central electrode, the lower casing and the condenser. The present invention is intended for use on the typical bushing and condenser arrangement and provides for wedging the condenser within the mounting flange such that the central electrode and the condenser will not fall into the electrical apparatus even upon failure of the upper weather casing. The present invention requires no modification of the typical bushing and condenser arrangement or of the mounting flange such as is required by the prior art arrangements discussed above. The present invention may therefore be implemented on existing bushings and electrical equipment.